1. Field of the Invention
The present invention relates to a solid oxide cell stack structure and a method for preparing same. In particular the invention relates to a method for producing a novel solid oxide cell stack structure by providing a catalyst precursor in the electrode layers after the stack has been assembled and initiated. The invention relates also to a method for reactivation of a solid oxide cell stack structure that has degraded during operation. The solid oxide cell stack structure is particularly suitable for use in solid oxide fuel cell (SOFC) and solid oxide electrolysing cell (SOEC) applications.
2. Description of the Related Art
Solid oxide cell (SOC) is the generic term for specific types of electrochemical cells, particularly solid oxide fuel cells (SOFC) and solid oxide electrolysing cells (SOEC) which in either case contain a solid electrolyte layer arranged in between two electrodes. One electrode acts as cathode and the other as anode. Usually, a solid oxide cell stack structure is formed by assembling a plurality of single cells into a stack and by providing internal and/or external manifolds. The manifolds enable the proper distribution of the reaction fluid into each single cell. The reaction fluid usually takes the form of an oxidant gas, such as air, and a suitable fuel, such as hydrogen. In SOFC the oxidant is in contact with the cathode while a fuel oxidised in the process is in contact with the anode. Conversely, in SOEC a fuel (like hydrogen or CO) is produced at the cathode by a reduction process from supplied species like steam or CO2, and oxygen is produced at the anode.
During preparation of SOC stack structures, single planar cells are stacked together with other components such as interconnect layers, current collectors and seals. Gas manifolds are subsequently adapted to the stack. After the stack has been assembled and provided with manifolds, the stack is subjected to a conditioning step, in which the stack is sintered by heat treating at temperatures usually above 800° C. and under pressure in order to tighten the seals and achieve electrical contact between the components of the stack.
A crucial part of the preparation of the final solid oxide cell stack structure is the provision of catalytic activity in the electrodes of the single cells forming the stack. Usually stacks are assembled by using electrodes where the catalytic material is an inherent part of the electrodes, for instance Ni/YSZ anodes and LSM/YSZ cathodes. Thus the electrodes are already active upon stacking and assembly.
Alternatively, the catalytic activity in electrodes can be provided in the stack manufacturing process itself rather than using already active electrodes. This can be done by impregnation of the individual electrodes with a suitable catalyst such as doped ceria prior to the sintering of the components of the cell and accordingly prior to the assembling of the cells into a stack.
This is for instance disclosed in EP-A-2031675 where the electrode (anode structure) is impregnated with a precursor solution of ceria which contains a solvent and a surfactant. After impregnation, one or more calcination steps in air is/are conducted at above 200° C. in order to form the desired oxides of ceria. Thereby nano-sized particles of ceria are provided in the electrode. Where an electrolyte is adapted to the electrode prior to the impregnation above, one or more sintering steps in a reducing atmosphere at above 1000° C. is also conducted.
In other instances, such as described in WO-A-2007/025762, the electrodes are impregnated after sintering the stack. Alternatively, the impregnation of the electrodes with a suitable catalyst is conducted after the sintering of the components, yet prior to the assembling of the components into a stack, thereby suppressing the negative effect on the performance exerted by unwanted reactions between the catalyst and the electrode material or unwanted reactions between the catalyst and the electrolyte material.
In either situation, an improvement in the anode and cathode performance is observed at these stages of impregnation at the production site itself, where impregnation is typically done on the cell as prepared, i.e. after the cathode firing step, so that the electrode becomes receptive to the impregnation. It is usually assumed that the electrode is most receptive for impregnation at this stage of production
However, it has been found that the beneficial effect of the catalyst in the electrodes is partly or fully lost upon heating the cells above 800° C. during stack assembling and initiation. Moreover, when subsequently put into operation, the catalytic activity in the electrodes of the stack structure is reduced over time due to for instance nickel agglomeration or sintering in the anodes or cathode degradation because of instability towards decomposition and impurities. This results in an overall decrease in the performance of the solid oxide cell stack structure. Such loss of performance can bring the stack structure outside the required specifications within a given application, for instance in SOFC stack structures used in power producing units, thus forcing their replacement. This is cost-intensive and deterring when considering the commercial prospects of using solid oxide cell technology.